1. Field of the Invention
The invention relates to a medium conveying apparatus.
2. Related Background Art
Hitherto, in an image forming apparatus such as printer, copying apparatus, facsimile apparatus, or the like, a medium conveying apparatus is arranged to convey a sheet as a medium (for example, refer to JP-A-8-11375).
The printer among the image forming apparatuses will now be described.
FIG. 2 is a front view showing a main portion of the medium conveying apparatus of the conventional printer. FIG. 3 is a side elevational view showing the main portion of the medium conveying apparatus of the conventional printer. FIG. 4 is a front view showing a state where the sheet has been inserted in the medium conveying apparatus of the conventional printer. FIG. 5 is a side elevational view showing the state where the sheet has been inserted in the medium conveying apparatus of the conventional printer.
In the diagrams, reference numerals 12 and 13 denote side frames of the printer; 14 an upper shaft; and 15 a lower shaft. The upper shaft 14 and the lower shaft 15 are supported to the side frames 12 and 13 through bushes (not shown). Reference numeral 16 denotes an upper guide and 17 indicates a lower guide. The upper guide 16 and the lower guide 17 are fixed to the side frames 12 and 13 in parallel with a distance δ by fixing means such as welding, screws, or the like.
Rollers 21 and 22 are arranged to the upper shaft 14 and the lower shaft 15 in a plurality of positions in the axial direction so as to be come into contact with each other. Tension springs 24 are attached to a plurality of portions of the upper guide 16 via brackets 23. Each tension spring 24 presses the upper shaft 14 toward the lower shaft 15 via a guide piece 26 by a predetermined urging force.
As shown in FIGS. 4 and 5, when a sheet 31 is inserted between the rollers 21 and 22, a proper pressing force is applied to the sheet 31 by the urging force and the sheet 31 is conveyed by a predetermined conveying force in association with the rotation of the rollers 21 and 22.
However, in the conventional medium conveying apparatus, when the sheet 31 comprises thick paper such as postcard, bankbook, or the like, as shown in FIG. 4, the upper shaft 14 is inclined and a variation is caused in conveying forces in the axial direction of the upper shaft 14 and the lower shaft 15. In this case, while a thickness of plain paper is equal to about 0.08 [mm], for example, a thickness of bankbook is equal to about 2 [mm]. Therefore, if the thick paper is used as a sheet 31, the sheet 31 is obliquely conveyed and cannot be stably conveyed.
To solve such a problem, there is a method of dividing the upper shaft 14 into two parts.
FIG. 6 is a front view showing a main portion of another medium conveying apparatus of the conventional printer.
In the diagram, reference numerals 12 and 13 denote the side frames of the printer; 32 and 33 upper shafts; and 15 the lower shaft. The upper shafts 32 and 33 are supported to the side frames 12 and 13 through bushes (not shown) in a cantilever beam manner. The lower shaft 15 is supported to the side frames 12 and 13 through bushes.
Reference numerals 21 and 22 denote the rollers, 24 the tension springs, and 26 the guide pieces.
In this case, since the upper shafts 32 and 33 are both independently supported, even when the sheet 31 as thick paper is conveyed, the upper shafts 32 and 33 are not inclined. Therefore, since no variation is caused in the conveying forces in the axial direction of the upper shafts 32 and 33 and the lower shaft 15, the oblique conveyance of the sheet 31 can be prevented and the sheet 31 can be stably conveyed.
However, since the upper shafts 32 and 33 are supported in the cantilever beam manner, the sufficient pressing force cannot be applied to the sheet 31 by the urging force of the tension springs 24. The conveying force for the sheet 31 cannot be sufficiently generated in association with the rotation of the rollers 21 and 22.
In the case of a printer constructed in such a manner that the sheet 31 is inserted along a guide arranged in a predetermined position on a table (not shown) of the printer, the upper shafts 32 and 33 can be independently arranged as mentioned above. However, In the case of a printer constructed in such a manner that the sheet 31 can be inserted into an arbitrary position in the width direction on the table, the upper shafts 32 and 33 are inclined in dependence on the position where the sheet 31 is inserted.